Thermostats have long been used to control HVAC systems. Zone sensors have also been used to provide signals indicative of a zone's status. The primary difference between a zone sensor and a thermostat is that the zone sensor provides a signal representative of a monitored condition, while a thermostat typically provides a command based upon the difference between the monitored condition and a setpoint. Since both zone sensors and thermostats each monitor a zone condition, they are considered interchangeable for purposes of the present invention.
Thermostats are typically mounted on a wall in the space or zone that is being heated or air conditioned at approximately four feet of height from the floor. The flow of air is impaired along the interior wall of a zone. Friction of the flowing air with the wall surface creates a boundary layer of air adjacent to the wall surface that impedes the passage of free zone air to the wall surface. This induces a significant delay in the thermostat sensing the actual room temperature. Additionally, the mass of the wall itself acts as a heat sink because it draws off some of the heat of the air passing over the wall. Accordingly, the thermostat is affected by air that is not truly representative, on a real time basis, of the air temperature of the zone. Clearly, a better location for sensing of the zone temperature would be an unobstructed portion of the zone where the heated or cooled air freely flows. In such a location, a sensor would receive an adequate supply of zone air and the air received would accurately reflect the temperature conditions of the air temperature in the zone in real time. Aesthetic and functional reasons, however, dictate that thermostats be wall mounted for most air conditional spaces and zones.
Consequently, there is a need to reduce the delay in sensing the actual zone temperature by a thermostat. A substantial delay in determining zone temperatures results in the HVAC system being out of synchronization with the temperature in the zone. The problem is exacerbated when changing the setpoint of the thermostat. This may occur in the morning, for instance, when the thermostat setting is changed, usually automatically, from an unoccupied setting to a more comfortable occupied setting. As a result of the lag in sensing the actual zone temperature, the HVAC system typically overshoots the new setpoint, causing the temperature to rise significantly above the new setpoint. This overshoot is both inefficient from an energy usage standpoint and is uncomfortable to the occupants of the zone. In many applications, it takes the overshoot several hours to dissipate before the temperature in the zone matches the setpoint.
As a partial solution to the above problems, thermostats have typically been made to protrude out into the zone. By moving the sensor out away from the wall, it was assumed that the sensor was free of the slowly moving boundary air along the wall and also free of the effects of the thermal mass of the wall. Aesthetic considerations, however, have dictated that the protrusion into the zone be minimized. Accordingly, present thermostats usually protrude only a small distance away from their supporting wall and into the zone.
The problem of inducing the flow of zone temperature air across a thermostat and through the protective cover of the thermostat has been a recurring issue within the industry. An early attempt to solve the problem with thermostats used in conjunction with pneumatic control systems is illustrated in U.S. Pat. No. 3,913,832. The '832 patent recognizes the need for accurate measurement of zone air temperature on a continual basis. Since the thermostat of the '832 patent already had a supply of air that was utilized in the pneumatic control system, the proposed solution to the air flow problem was to induce the flow of zone temperature air by using a T-shaped tube. The T-shaped tube had pneumatic supply air flowing in it and utilized that flow to create a venturi effect to draw a flow of zone temperature air into the thermostat housing.
A second idea to ensure that the ambient air temperature was continually being sensed is detailed in U.S. Pat. No. 4,141,496. As in the '832 patent, the '496 patent deals with the problem of temperature sensing in a pneumatically controlled system. The compressed pneumatic control air in the '496 system is directed through a nozzle to create a suction in a line downstream of the nozzle. The line was introduced into a chamber, creating a partial vacuum in the chamber. The chamber contained the sensor and had an opening into the zone. The vacuum drew ambient air into the chamber to be sensed by the sensor. Both the '832 patent and the '496 patent rely on the availability of pressurized control air and are not applicable to HVAC control systems that are not pneumatically controlled.
With the advent of electronic thermostats, there has been no adequate means of inducing flow to the sensor. U.S. Pat. No. 4,659,236 addresses the problem of dissipating the heat generated by electronics integral to a thermostat housing. The '236 patent acknowledges that the two approaches to improve the accuracy of a thermostat's electronic temperature sensor, e.g., having the thermostat project further into the zone and using the wall as a heat sink, have not been successful. The solution proposed by the '236 patent is to incorporate a thermal shield between the heat generating components and the temperature sensor. While the shield may minimize the effect of heat build up caused by the electronic components, it does not address the problem of air flow across the sensor.
Isolation of heat generating elements from the sensor of a thermostat is also evident in U.S. Pat. No. 4,347,443. The '443 patent relates to electronic thermostats and discloses a projecting, room thermostat that includes an air-wash through-passage for sensing room temperature. The air-wash also removes the heat generated by the electronic components. The '443 design requires a mounting for the thermostat with a space between the back side of the circuit board and a cover to allow airflow over the circuit board. A shield on the front side of the circuit board is required to shield the temperature sensor from the heat generated by a power supply circuit and a signal output circuit. The '443 patent relies on convection for air flow across the sensor.
While each of the above-described patents recognize the problem of requiring a flow of ambient zone air across the sensor of a thermostat, the solutions proposed rely on either the pressure of compressed air in pneumatic thermostats or on passive heat shields used in combination with convective air flow in electronic thermostats. A thermostat which presented a very shallow depth for aesthetic purposes, but was continually, actively supplied with an adequate sample of zone air under all conditions of air movement, without requiring the availability of pneumatic air, or without relying primarily on convective air flow, would provide decided advantages.